The present invention relates to an image recording device of electrostatic transfer type with improved tone reproduction capability and, more particularly, to a colored image recording device with improved original-versus-copy density relationship, which offers a wide range of gradient linearity in .gamma.(gamma)-value and is especially suitable for the reproduction of originals having not only highlight and shadow areas but also half toned areas.
More than 40 years has passed since the principle of electrophotography of practical use was first introduced by Carlson. In an image recording device according thereto, like an electrostatic or xerographic copier, a series of processes are performed, such as a uniform electrostatic charge imparted on a photoconductive insulating surface, a local charge dissipation giving a latent image corresponding to a light pattern, a development with colored developer visualizing the latent image, a transfer of the visualized image to a recording sheet and a fixation to obtain the recording sheet with the transferred image.
Needless to say, tone reproduction with complete fidelity is preferred. In terms of gradation, a linear relationship between the density of the original manuscript (OD) and that of the copied output (CD) is necessary. However, the electrical potential of conventional photoconductive material undergoes a decay in a fashion far from ideal and, as shown in FIGS. 6a, 6b and 6c, a linearity in the OD-CD relationship is unavilable, particularly with a higher OD, where the CD saturates to a certain level and the original gradation cannot be reproduced.
Particularly in a multichromatic "full color" copier, a saturation in one color results in an unbalance in hue to give a different color from the original. As shown in FIGS. 6a, 6b and 6c, wherein the development bias potential, the amount of exposure and the charge voltage of the photoconductive layer, respectively, are modified, the OD-CD relationship, such as contrast and saturation, can be adjusted in various ways. However, these Figures tell that, with any of these modifications of such parameters, a linear relationship can be maintained in a relatively small range only. Accordingly, many kinds of parameter modification means are provided with conventional copiers, which are to be controlled to give the best possible tone reproduction relating to the particular type of the original image. Many skillful tests are necessary for this purpose, but in spite of these a continuous tone illustration like a photograph cannot be reproduced satisfactorily even by well-trained staffs, because of its wide range in gradation.
Photographic density or optical density is a degree of opacity. It is the ratio of the intensity of light projected to the image in dispute to the intensity of light which has passed through the image. Practically, however, the density D.sub.R is defined as: EQU D.sub.R =log (Rw/R)
wherein R is the intensity of light reflected to the perpendicular direction when certain amount of light is projected from the 45.degree. angle to the image and Rw is the reflection measured for a white sheet in the same manner.